The present invention relates to a process for the controlled production of high density polyethylene (HDPE) or of linear low density polyethylene (LLDPE) in one or more gas phase reactors, in the presence of either chromium or Ziegler-Natta catalysts, said process possessing an on-line control of certain process variables as well as the control of certain physical properties of the final resin product. More specifically, such process comprises: i) the use of models for inferring the physical properties as well as the process variables which are not measured continuously and ii) models which are relevant for the control of said properties and operating conditions of the process under study. The control of the process variables provides further the maximization of the production rate as well as of the catalyst yield in the polymerization reaction.
The control of the process variables in petrochemical plants is normally manually effected by operators who periodically sample the product to be tested and act to maintain or to correct the operating conditions as to obtain a product having the desired characteristics. This involves delays related to required corrections, since the sampling and laboratory tests normally lag the on-line process, besides possible human errors. Further, the dynamics of petrochemical processes is generally slow. Thus, long periods of time may be required so that the effect of the adjustments performed on the process input variables become effective. Therefore, the slow dynamics is a feature which renders difficult the control of the industrial unit since the worker may not know if there has been a sufficient time for the effects of the performed adjustments to be apparent or either if more time is required so that said effects are completely perceived.
On the other hand, techniques for the empirical as well as rigorous modeling are known, which may be used for obtaining process models. Such mathematical models are able to infer the value of certain process variables that are periodically measured from other process variables that are continuously measured. Besides, the mathematical models may also be used to predict the future behavior of process variables caused by modifications in the operating conditions of an industrial plant.
The techniques of rigorous modeling are based on the physical principles, which describe the basic interactions between the process variables. As compared to the empirical techniques, the rigorous models are more difficult to construct and require a deep knowledge of the process. Besides, the complexity of the equations, which make up the rigorous model, may render it, at least in some cases, unsuitable for on-line implementation, this difficulty arising from the long time to resolve such equations, even making use of computers.
On the contrary, the techniques of empirical modeling do not require such a deep knowledge of the process being modeled and originate simpler mathematical models which may be quickly executed, being therefore suitable for being executed in real time. A disadvantage of the empirical models is that they cannot be used under operating conditions different from those used in their identification. Models obtained from linear and non-linear regressions as well as neural networks are among the relevant empirical modeling techniques described in the literature.
Neural networks are networks of either neurons or elements which are interconnected in a unique way. Typically, the networks are made up of input neurons which receive signals or information from outside the network, output neurons which transmit signals or information to outside the network and at least one intermediate layer of neurons which receive and transmit the information to other neurons.
Besides the fact of a model being rigorous or empirical, it may also be characterized as static or dynamic. A model is said static when it yields as a result the steady state values of the process variables in view of the values assumed by the input variables of the system. On the other hand, the dynamic model presents, besides the information on the steady state of the system, information on the way by which the output variables move between two steady states.
A large number of references describe processes provided with control, for the production of polyolefins.
U.S. Pat. No. 3,636,326 teaches how to adjust the production rate of a polymerization reactor based on the catalyst yield calculated in real time. Thus, possible modifications in the catalyst yield may be automatically compensated by means of a feedback control loop. This kind of control may be practiced when the variables which affect the catalyst yield are not known or not measured. In this case the best to do is to automatically adjust the catalyst rate as soon as a variation in the catalyst yield is perceived. However, by using process models it is possible to preview that catalyst yield will undergo modifications due to certain changes in the operating conditions. Thus, it is possible to previously adjust the catalyst rate so that the change in catalyst yield does not significantly affect the production rate of the reactor.
This and other advantages to be mentioned hereinbelow are features of the controlled process to be described and claimed in the present application.
U.S. Pat. No. 3,998,995 teaches a process in which the production rate and monomer concentration are controlled in a polymerization reactor able to restrict the concentration of the main monomer and the solids concentration at maximum values. In said process, the production rate is controlled by the flow rate of olefinic monomer while the concentration of monomer in the reaction zone is controlled through the adjustment of a diluent feed fed to the process. If the maximum limiting value for the monomer concentration is reached, monomer flow rate is then adjusted to reduce said concentration and no longer to control production. If the maximum limiting value for the solids concentration is reached, the flow rate of diluent is adjusted to reduce said concentration and no longer to control the monomer concentration. Whenever the constraints are violated the goal of controlling production rate and monomer concentration is abandoned in favor of the continuity of operation. The control system which is unable to deal with constraints will not be able to guarantee the safety and the operational continuity required for an industrial unit to accomplish its goals. In spite of the fact that the said U.S. Patent presents a control procedure able to deal with the plant constraints, the system works only from the moment when the constraint has been effectively violated. A controller which could predict the future behavior of the process variables would be able to promote the required adjustments so that the constraint would not be violated or would be just slightly violated. A process whose constraints are so controlled, would allow that the main control objectives would not be abandoned.
A method for the control of the intrinsic viscosity of a polymer which is being produced has been taught in U.S. Pat. No. 3,878,379. The technology unveiled in said U.S. patent, besides being specifically directed to the production of polyethylene terephthalate, relates to one single variable and does not contemplate the control of different polymer properties.
U.S. Pat. No. 4,469,853 provides a process for the production of polyolefins in the liquid phase having a few well defined properties. Such process describes the use of chromatography for the measurement of the concentrations of olefin monomers and hydrogen in the gaseous phase, which is formed on the top of the reactor. The control of the flow rates of ethylene and comonomers is adjusted so as to keep constant the ratio between the concentrations of said reagents so as to lead to a polyolefin having predetermined density. Further, said U.S. patent achieves the control of the ratios between the concentrations of hydrogen and main monomer by adjusting the flow rate of hydrogen feed to the reactor and thus leading to a polyolefin having a predetermined melt flow rate. Optionally, the partial pressure of ethylene in the reactor could be controlled through the adjustment of the flow rate of the feed of catalyst while the pressure could be controlled through the adjustment of the reactor flow rate of purge. The problem addressed by the cited application is the control of reagent concentrations in the reactor, which is a gas/liquid phase reactor. The reference proposes that the control of the concentration in the reactor may be improved by monitoring reagent concentrations in the gas phase instead of the liquid phase such improved concentration control leading to a polymer with more stable properties. The control strategy cited in the referenced invention is that of simply using hydrogen to control a flow property and comonomer to control polymer density. The problem with such a strategy is that these control loops are coupled since comonomer also affects the flow properties and hydrogen also affects density. The invention as claimed herein teaches how to solve the loop interaction problem by using a multivariable control strategy.
U.S. Pat. No. 5,098,967 teaches how to control the molecular weight of polypropylene and its copolymers produced in the liquid phase by measuring the heat released during the polymerization. The calculation of the amount of polymerized monomer is effected on the basis of said measurement. This leads to the possibility to anticipate a controlled flow rate of hydrogen feed to the reactor so as to keep a predetermined ratio between reacted monomer and hydrogen feed so that it is possible to obtain a polymer of desired molecular weight.
U.S. Pat. No. 5,504,166 teaches how to control the melt flow rate and the comonomer content of a polymer produced in a horizontal stirred-bed reactor. Such properties are related to a set of operation variables and to parameters which are intrinsic to the process.
U.S. Pat. No. 5,282,261 teaches the use of values which are predicted by a neural, network in real time to be used instead of a measuring instrument or a lab test as an input for a controller, so as to implement an inference and control system in a continuous process while using neural networks. However, said U.S. patent does not teach how to apply the described system to polymerization process nor teaches any control structure which might be particularly applied to the process for producing polyethylene and its copolymers.
U.S. Pat. No. 5,844,054 discloses a process for the polymerization of olefins in gas phase fluidized bed reactors comprising control of a set of controlled variables comprised of production rate, melt flow rate and density through predictive computer models and coordinated adjustments to a set of manipulated variables comprised of catalyst feed rate, oxygen to alpha-olefin molar ratio and comonomer concentration. The invention as claimed herein provides improved functionality over the cited reference application as it takes important influences such as hydrogen concentration, inert concentration, fluidized bed level, fluidized bed temperature, pressure, recycle gas flow rate, bleed stream flow rate into account for predicting the behavior of process variables and product properties. Further, the present invention also discloses how to achieve improved process stability as this broader set of input variables is simultaneously monitored and coordinated to provide not only control of production rate and melt flow ratio control but also of melt flow rate ratio, catalyst productivity, superficial velocity, ethylene partial pressure. hydrogen concentration and comonomer concentration.
Thus, none of the patents no the publications of the state-of-the-art technique, either isolated or combined to each other, teach a process for producing HDPE or LLDPE in a single or combined gas phase reactors, using chromium or Ziegler-Natta catalysts in which process may be on-line and simultaneously controlled the production rate, the catalyst yield, the superficial velocity and the gas composition within the reactor, among other process variables, together with various physical properties of the product, such as melt flow rate and density, such process being described and claimed in the present application.
The general object of WO 93/24533 is to provide an improved method for the advanced control for the continuous gas-phase polymerization of an alpha-olefin in a substantially horizontal, quench-cooled, stirred bed reactor in which solid liquid and gas phases are present. The invention as claimed by the Applicant is directed towards a process which differs from the cited reference in a key aspect, this difference arising from the fact that the type of reactors used in each process are different. The process variables relevant for the control of a quench-cooled, stirred bed reactor are not the same as the ones which are relevant for the control of a reactor containing only gas and solid phases and, thus, the reference neither discloses nor suggests which manipulated variables and disturbances should be considered for the advanced control of the process of the present invention.
WO 98/54231 relates to improved control of the temperature of a fluidized bed by the simultaneous and coordinated manipulation of the water flow rate and the cycle gas flow rate. Thus, the reference discloses a process in which temperature is the controlled variable and the water flow rate and the cycle gas flow rate are the manipulated variables. Differently, the process claimed in the present invention relates to improved control of a set of controlled variables comprising production rate, MFR1, MFRR, density, ethylene partial pressure and concentration of comonomers among other variables. The invention as claimed hereinafter makes no reference to any method whatsoever for improving the temperature control of the fluidized bed.
The present invention relates to a process for the controlled production of HDPE or LLDPE in a single or combined gas phase reactors, under polymerization conditions, in the presence of hydrogen, oxygen, inert diluents and using chromium or Ziegler-Natta catalysts, the process having an on-line control of certain process variables, as well as of some physical properties of the produced resin.
Such control is based on dynamic mathematical models which describe the effects of the manipulated variables and of the disturbances on the controlled variables as well as the variables whose values should be restricted to certain operation ranges. The use of such models renders possible to infer the properties of polyethylene in real time from the on-line measurement of other process variables. Besides providing an estimate of the quality of the polymer, models are used by an optimization algorithm which determines the best set of control actions to be taken so that the controlled variables can approach the set point for each of the variables without violating the constraints imposed to assure continuous and safe operation of the unit.
Thus, the present invention provides a process for the controlled production of polyethylene and its copolymers, the control being based on dynamic mathematical models used for the simultaneous and on-line line control of the melt flow rate 1 (MFR1), the ratio of melt flow rates (MFRR) and the density of the resin produced from the chromium catalyst as well as the melt flow rate 2 (MFR2) and the density of the resin produced from Ziegler-Natta catalysts.
The present invention still provides a process endowed with an on-line control of the production rate, the catalyst productivity, the composition of the gas within the reactor, the superficial velocity of the fluidized bed system, the bubble temperature of the reactor recycle stream and the difference between the inlet and exit temperature of the water in the cooling system of the reactor.
The present invention provides further the maximization of the production rate of polyethylene and the catalyst yield for the described process.